c# - 如果字符串崩溃，则 C# 自定义错误

Question

所以我在 c# 中有以下字符串，它返回一个 mysql 查询。

string id = cmd.ExecuteScalar().ToString();

问题是如果这个字符串的输入无效，它会使应用程序崩溃。这不应该使应用程序崩溃，而是显示：

MessageBox.Show("Your key (" + KeyNr + ") was invalid or not active. \nPlease check if it is the correct key or you need to buy another month.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

尝试尝试并没有运气。有人可以帮忙吗？

我的一些尝试：

try
{
   string id = cmd.ExecuteScalar().ToString();
}
catch (Exception ex)
{
   MessageBox.Show("Your key (" + KeyNr + ") was invalid or not active. \nPlease check if it is the correct key or you need to buy another month.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

和

try
{
   string id = cmd.ExecuteScalar().ToString();
}
catch (catch (MySqlException))
{
   MessageBox.Show("Your key (" + KeyNr + ") was invalid or not active. \nPlease check if it is the correct key or you need to buy another month.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

一切都给了我：

Error   2   The name 'id' does not exist in the current context

Answer 1

在不为您解决的情况下，您需要cmd.ExecuteScalar()在假设它执行并可以转换为字符串之前存储并检查结果。

var result = cmd.ExecuteScalar();
if (/* result is valid */){
  id = result.ToString();
} else {
  MessageBox.Show(/* message */);
}

尽可能远离异常检查，因为使用它们会不必要地增加性能，并且（在很多方面）是一种跳过良好老式检查的懒惰方式。

Answer 2

试试这段代码：

string id = string.Empty;

try
{
   id = cmd.ExecuteScalar().ToString();
}
catch(Exception ex)
{
    MessageBox.Show("Your key (" + KeyNr + ") was invalid or not active. \nPlease check if it is the correct key or you need to buy another month.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
}

您甚至可以将其捕获到更具体的 Exception 而不是 generic Exception。